In power supplies having push pull drive into a transformer, the transformer can be fully used between the positive maximum flux and the negative maximum flux of the hysteresis curve -.phi.m to +.phi.m. However, in single-ended drive into the transformer, only the positive portion of the hysteresis curve 0 to +.phi.m or even less than the full positive portion because of the remanent flux (.phi.r). In this case, the transformer can only be used or driven between +.phi.r and +.phi.m which is approximately one-third the magnitude of the path from -.phi.m to +.phi.m in push-pull operation. Thus, for the same output power and the same kind of ferrite, single-ended forward drive requires a transformer which is about three times larger than that required for push pull drive.